Radiosensitization by the halogenated pyrimidine iododeoxyuridine (IdUrd) has been demonstrated in laboratory studies and is directly related to the percent thymidine replacement in DNA. This phase I study has been designed to assess the clinical feasibility of IdUrd as radiosensitizer when given with accelerated hyperfractionated radiotherapy (XRT) in the management of high grade gliomas. The IdUrd will be given as a 24 hour infusion over 28 consecutive days. The radiotherapy will be 60 Gy at 1.2 Gy per fraction twice a day for fifty fractions with concomitant, reduced volume boost of 10 Gy at 2 Gy per fraction once a week for five fractions.